1. Technical Field of the Invention
This invention is in the field of cryocoolers, and more particularly in the field of regenerative cryocoolers.
2. Background of the Related Art
Multi-stage cryocoolers are of fundamental interest for many applications in which cryogenic cooling is required. For example, some applications require the simultaneous cooling of two objects to cryogenic, but different, temperatures. In the case of a long wave infrared sensor, for instance, the focal plane assembly may require an operating temperature of around 40 K, while the optics may need to be maintained at a different temperature, such as about 100 K. One approach for such situations is to use a single-stage cooler and extract all of the refrigeration at the coldest temperature. However, this is thermodynamically inefficient. Another approach is to use two single-stage cryocoolers with one each at the two temperature reservoirs. This approach has the disadvantage of being expensive and large in size. A better approach that has been done in the past is to use a two-stage cryocooler with the first-stage cooling the higher operating temperature component, and the second stage cooling the lower operating temperature component. Multi-stage cryocoolers are generally more efficient than single-stage coolers, because a portion of the internal parasitic thermal losses can be removed from the system at higher temperatures, thus producing less entropy generation.
FIG. 1 shows a portion of a prior art cryocooler 10. The cryocooler 10 includes a compressor 11 that is coupled to a first-stage Stirling expander 20 with a first-stage regenerator 21, a plenum 22, and a piston or displacer 23. The piston 23, which contains the regenerator 21, oscillates within a cold cylinder 25. A wall of the cold cylinder 25 provides first stage pressure containment and thermal isolation from the ambient warm end. The plenum 22 and a motor assembly 27 are contained within an expander housing 26. The first-stage expander 20 also includes a first-stage heat exchanger 24 in a first-stage manifold 28. The piston or displacer 23 is used to expand the working gas, such as helium, downstream of the regenerator 21 such that refrigeration is produced in the first-stage heat exchanger 24. The working gas absorbs the first stage heat load from the environment as it passes through the first-stage heat exchanger 24. The first-stage heat exchanger 24 is in pneumatic communication with a second-stage pulse tube expander 30, where the (colder) second-stage refrigeration is produced. The pulse tube expander 30 includes a second-stage regenerator 31 and a pulse tube 32. The second-stage regenerator 31 and the pulse tube 32 may be generally parallel to one another, forming legs of a U-shaped configuration. The second-stage regenerator 31 and the pulse tube 32 are linked together by a flow passage 36 in a second-stage manifold 41. The flow passage 36 links a downstream end of the second-stage regenerator 31 with an upstream end of the pulse tube 32. End caps 42 and 43 close off the respective ends of the second-stage regenerator 31 and the pulse tube 32, within the second-stage manifold 41. A second-stage cold heat exchanger 44 is at an upstream end of the pulse tube 32, in the second-stage manifold 41. A second-stage warm heat exchanger 46 is at a downstream end of the pulse tube 32, in the first-stage manifold 28. The cryocooler 10 may be used to cool objects thermally coupled to either or both of the manifolds 28 and 41. Objects in thermal communication with the first-stage manifold 28 are cooled at a first cold temperature, and objects in communication with the second-stage manifold 41 are cooled at an even lower cold temperature. Further details regarding prior art cryocoolers may be found in commonly-assigned U.S. Pat. Nos. 6,167,707, and 6,330,800, the descriptions and figures of which are incorporated herein by reference.
In installation of the prior art cryocooler 10, the cold cylinder 25, the first-stage manifold 28, and the second-stage pulse tube expander 30 (collectively a cold head 50) are often required to be supported only at the expander housing 26. This leaves the second-stage pulse tube expander 30, the second-stage manifold 41, the first-stage manifold 28, and much of the cold cylinder 25, cantilevered off of the housing 26. This has caused difficulties, particularly in space flight applications, where the cooling system must be able to withstand loads and random vibrations generated during launch.
From the foregoing it will be appreciated that improvements in multi-stage cryocoolers may be possible.